Thermal imaging or thermography is a recording process wherein images are generated by the use of image-wise modulated thermal energy.
Two different methods are known. In accordance with a first method, referred to as direct thermal imaging, a visible image pattern is obtained by direct image-wise heating of a recording material containing matter that by a chemical or physical process changes colour or optical density. A particularly interesting direct thermal imaging element comprises an organic silver salt in combination with a reducing agent. When heated, the silver ions are developed to metallic silver.
In accordance with a second method, referred to as thermal dye transfer printing, a visible image pattern is formed by transfer of a coloured species from an image-wise heated donor element onto a receptor element. Thermal dye transfer printing is a recording method wherein a donor element is used that is provided with a dye layer. Portions of the dye are transferred onto a contacting receiver element by the application of heat in a pattern normally controlled by electronic information signals.
Image-wise heating can in either of the above systems be obtained by means of a thermal recording head comprising a plurality of juxtaposed resistors. Most commonly line-wise recording heads are used. The recording head comprises one resistor per pixel in a line. The head writes one line at the time. A two-dimensional image is then built from a large number of one-dimensional scan-lines.
In either of these techniques, the heating is controlled by an electric signal. The electric signal represents the density value of each pixel of an image by means of an N-bit digital signal value. The signal value of a pixel is transferred through a driver circuit to a thermal recording head.
The elements of a thermal recording head commonly are binary controllable devices, i.e. they offer only on/off control. So, when an N-bit signal value is to be transferred to an element of a recording head, a time-multiplexing scheme is required to translate the N-bit pixel value into consecutive 1-bit values that can be consecutively fed to an element of the thermal head to activate its operation during successive time steps. Such a process is commonly called `slicing`. In every time step or time slice, new pixel information is generated and transmitted to the thermal head. Then, an enable signal (called strobe signal) is generated to turn a selected pixel momentarily on.
The prior art slicing procedure is summarized in the following with regard to the process of imaging a density D represented by an N-bit value `A`:
1) An N-bit value `A` is converted into a sequence of M 1-bit values a.sub.1, a.sub.2, . . . a.sub.M ; wherein M is less than or equal to .sup.2 N and the conversion is such that the sum of all 1-bit values a.sub.1, a.sub.2 . . . a.sub.M is exactly equal to A. PA1 2) All 1-bit values a.sub.1, a.sub.2, . . . a.sub.M are generated in sequence, each for the same duration of time t, the sequence of 1-bit values constitutes a signal, that is active during a period of time t.sub.on that is equal to t multiplied by A, since the sum of a.sub.1, a.sub.2, . . . a.sub.M equals A. PA1 1) Small time steps are more difficult to generate very accurately than larger time steps. PA1 2) The ratio of the largest acceptable t.sub.on time (further called t.sub.on,max) and the smallest non-zero t.sub.on time (called t.sub.on, min), determines the number of bits N needed to represent a value, for example the digital N-bit representation of a density D. So, in order to be able to reduce t.sub.on time, a larger number of bits is required. Normally t=t.sub.on, min, and N is such that 2.sup.(N-1) &lt;=t.sub.on,max &lt;2.sup.N. PA1 3) Smaller time steps not only result in larger values of N, but also in an increase of information to be transferred to the thermal head when producing an image. PA1 (i) converting said N-bit value A into a linear combination ##EQU1## of P values A.sub.i, P being greater than or equal to 2 and being smaller than N, wherein with each of said values A.sub.i a predefined time step t.sub.i corresponds, and PA1 (ii) converting said digital signal into a binary signal that is active during a period t.sub.on, wherein t.sub.on is equal to ##EQU2## PA1 t.sub.1 =K * t PA1 t.sub.2 =(K+1) * t PA1 0&lt;=X.sub.2 &lt;=K-1 PA1 the lower limit K*(K-1) mentioned above, increases quadratically with K, reducing the usable range of the N bit data values X. The range of values that can be exactly represented is reduced from (0, 2.sup.N -1) to (K*(K-1), 2.sup.N -1). PA1 the total time needed to generate all 1-bit data values increases with increasing K-values. The total time to be allotted for a worst-case data value X has an upper boundary of T.sub.max =X.sub.1,max *t.sub.1 +X.sub.2,max *t.sub.2 PA1 a number of individually energisable recording elements, PA1 converting means for converting a digital signal representative of said N-bit data value representing a density value D into abinary signal that is active during a time period t.sub.on, PA1 means for applying said binary signal to a recording element to activate its operation, characterised in that said converting means convert said digital signal into a binary signal that is active during a time period t.sub.on that is equal to the sum of a number of time steps being selected from at least two different predefined time steps.
The constant time period t in the slicing process determines the smallest achievable non-zero t.sub.on time. This period is called the time step of the slicing process.
Small time step values allow very short and precise t.sub.on times.
However, the use of a very small time step also has disadvantages:
So, reducing t.sub.on,min for example by a factor of two, while keeping t.sub.on, max constant will cause an increase of N by 1.
Let us assume a typical case wherein a pixel value is represented by a 10-bit digital value and wherein pixel values are fed to a line-wise thermal head comprising about 4350 individual addressable elements. Let us further assume that one shift register is used for each group of 128 individual elements out of these 4350 elements. (Further details on the use of such shift registers will be explained further on with reference to the drawings).
Conversion of the 10-bit digital value for each pixel into consecutive 1-bit values causes a hundred fold increase of the data to be generated and transferred to the thermal head. Indeed, slicing causes a 10 bit value to be converted into 2.sup.10 =1024 one-bit values that are generated in each of the slices whereas without this conversion 10 bits were required only once per scan line.
So, slicing is a procedure that involves huge amounts of data, requiring very high clock frequencies or massive parallel hardware.
For example, in the case in which 20 msec per scan line is provided, each of the above-mentioned slices has less than 20 microseconds available to generate and transmit 1 bit to every element of the thermal head.
Assuming that there are about 4350 individual elements in the thermal head, this amounts to a bit generation requirement of around 220 Mbit/sec.
In the described example a thermal head comprises a shift register per 128 individual elements, so 34 shift registers are used. Then, each shift register should be clocked at 6.4 MHz. This frequency comes very near to the maximum performance specification of these shift register chips.
In thermal printing technology currently under development, one even wishes to reduce the line printing time, e.g. to 10 msec per scanline instead of 20 msec per scanline. Then, frequencies double (or the degree of parallelism doubles) and the shift frequencies inside the head clearly exceed the head's capabilities.
In U.S. Pat. No. 4,335,968 a thermal printing method has been disclosed wherein a colour tint is reproduced by delivering the bits of a digital value representing a tint in descending order during different periods of time. For example bits of weight 0, 1, 2, 3 are delivered by registers M.sub.i during respective times t.sub.1, t.sub.2 -t.sub.1, t.sub.3 -t.sub.2, t.sub.4 -t.sub.3. A power supply delivering power to a number of resistors, delivers a feed voltage when the corresponding input is in a logical state `1`. In this method the period of time during which power is applied to a resistor element is determined by the weight (bit position) of the significant bits in a digital representation of a tint to be printed.
In GB 2 196 498 a pulse width modulation control circuit has been described for thermal printers. A digital representation of a density value is split into an upper bit and a lower bit portion. The upper bit portion is converted into a main pulse width modulated signal based on a first time duration. The rest of the bits are converted into a sub pulse-width modulated signal based on a second unit time duration. Head driving signals are generated on the basis of the main and sub pulse width modulated signals.